In a process of separation by chromatography, the efficiency of the column used is a key parameter. The column is generally filled with a solid product in the form of very fine grains usually of a size comprised between 5 and 100 μm, forming the chromatographic bed. In order to obtain increased efficiency, the arrangement of the grains inside the column must be as homogenous as possible, and, moreover, empty volumes between the chromatographic bed and the column inlets-outlets must be avoided. It is known in the prior art, to resort to dynamic axial compression columns in order to ensure the existence of a compact bed, without free space between the inlet-outlet distributors and the bed.
Thus, from the French Patent Application FR-A-2 219 797, a chromatographic device is known, being presented in the form of a column intended to contain a bed of adsorbent material. The device is constituted by a tube comprising a lid and a element sliding in the tube, allowing a pressure to be exerted in the tube. The sliding element is a piston comprising at its head a porous plate which is permeable to fluids. In order to obtain the bed intended for chromatography, a suspension of particles of material capable of constituting an adsorbent bed is introduced into the tube with its lid removed, the piston being moved back sufficiently to allow the introduction of the suspension, without necessarily being at the dead centre. The lid is fitted to the tube, then a pressure is exerted on said suspension by means of the piston. The fluid is forced back through the porous plates, and said particles are compressed between the piston and the lid. This device makes it possible to achieve, in one embodiment, the compression stage of the chosen bed, and the desired compression of the particles when the sliding element has travelled ⅖ of the height of the column.
This device however has the disadvantage of inaccurate compression of the bed, since it is evaluated only by the position of the piston along the tube.
Also known from the European Patent Application EP-A-0 145 578, under priority of the Applications FR-A-2 556 099 and FR-A-2 573 532, is a chromatographic device comprising a tube, one part of which is intended to contain an adsorbent bed. This tube comprises two end walls and a piston sliding longitudinally in the tube. The piston marks, between the two end walls, a separation between one chromatographic chamber containing the bed and an enclosure in which a pressure prevails, such as to move the piston along the tube. The special feature of this device is that the fluid contained in the enclosure is liquid to be chromatographed diverted from the input conduit of the latter. Part of the liquid is diverted towards the enclosure via an enclosure conduit and the other part of the fluid is carried along to the chromatographic chamber via a flow conduit crossing the enclosure and the piston. When the piston is in the equilibrium position, the force exerted from the enclosure side on the piston is equal to the sum of the force on the piston produced by the flow of the liquid into the chamber to be chromatographed, and the mechanical force of the bed on the piston.
This device makes it possible, apart from carrying out the elution stage, to define the pressure exerted on the bed.
In a first embodiment, the piston has a constant section but the pressure in the flow and enclosure conduits is controlled by flap valves. Thus the valves can be calibrated such that the difference between the pressures exerted by the flow of the liquid and by the liquid in the enclosure are adjusted to a set-point pressure on the bed.
In another embodiment, the pressure in the flow and the enclosure conduits is the same, but the piston section varies. The piston is in two sections sliding in corresponding bores, the smaller section A being situated on the chromatographic chamber side and the larger section B on the enclosure side. Due to the difference in the areas of the piston surfaces A and B, the force exerted at B is greater than the force exerted at A. The piston sections can thus be chosen so that the difference between the pressures exerted by the flow of the liquid and by the liquid in the enclosure is adjusted to a set-point pressure on the bed.
In fact, the device described in the above document has various disadvantages. If a drop in pressure occurs in the flow conduit, the pressure exerted on the piston by the liquid in the enclosure is applied directly to the bed, which can give rise to deterioration of the bed. Moreover, the movements of the chromatographic bed not being monitored and the enclosure not being able to move back liquid during the chromatography, this device cannot adjust the pressure exerted on the bed as a function of the movements of the latter. A deterioration of the bed can again result. On the other hand, this system cannot be used in an SMB (Simulated Moving Bed) or VARICOL. In such configurations, the chromatographic devices are mounted in a loop and the liquids injected into the beds alternate. In the device described in EP-A-0 145 578, the same liquid is sent into the bed and into the enclosure, the alternation of injection of liquids gives rise to a mixture of these.